Over forty outbreaks of P. ramorum have occurred
in managed gardens in the UK. Three of these sites, one in the south-east
of England and two in the south-west, have been closely monitored since
October 2003. These sites represented differing disease scenarios at the
start of monitoring, as eradication action had already taken place at
the site in the south-east, whereas no action had taken place in the south-west.
The progress of existing infections and the development of new infections
were monitored at all sites. A number of current or previously infected
areas within the gardens were selected for experimentation and marked
out in metre square grids to cover at least 3m in all directions from
the original infected plant. Soil and leaf litter samples, taken each
month, were analysed for presence of P. ramorum by isolation
and microscopic examination of the cultures, and any positives were confirmed
by TaqMan® PCR. In areas of high infection, sequential sampling was
carried out through the soil profile to examine the depth of contamination.
Baits were deployed along all watercourses on a three-monthly basis to
investigate extent of contamination in ponds and streams within the gardens
and to quantify the effect of seasonal variation on inoculum levels. The
influence of environmental factors on timing and quantity of spore release
were investigated using volumetric spore samplers, rain-water traps and
logging of meteorological data by weather stations. Sampling strategies
have successfully established the level of contamination at each of the
sites and have shown that the pathogen can survive winter conditions in
the south of England within plant material and in the wider environment.
Monitoring of the selected sites revealed different levels of infection
in terms of numbers of plants affected and degree of soil contamination,
with higher levels of infection and contamination in the south-west sites.
No direct evidence of significant vector involvement in pathogen dispersal
was found, but a number of pathways used by members of the public were
found to be infected in the south-east site. The seasonal baiting of water-courses
at this site have indicated widespread contamination of streams, ponds
and lakes. However, significantly lower levels of P. ramorum
contamination were detected in summer compared to the spring. This may
reflect a general reduction in inoculum as a result the earlier eradication
strategies adopted but may also indicate less favourable conditions for
sporulation and spread during the summer. There have been no new plant
infections at the south-east site (other than on regrowth) and levels
of soil and leaf litter contamination are low. This also suggests that
the strategy of early removal of infected plants and surface leaf litter
has been successful in reducing disease inoculum and spread.